SALT LAKE CITY (AP) — Ozone pollution levels that can be harmful to people's lungs exceed federal thresholds along a stretch of Utah's most-populated metropolitan areas in the summer and in the winter in a rural area where oil and gas operations are prevalent, Utah state regulators said Tuesday.

The U.S. Environmental Protection Agency sent Utah a written warning, giving the state three years to reign in the pollution, said Bryce Bird, director of the Division of Air Quality with the Utah Department of Environmental Quality. He said state officials are already working to reduce ozone levels, but if they don't drop enough in three years an official action plan will be required.

The pollution was identified in all or part of Salt Lake, Davis, Weber, Tooele and Utah counties along the Wasatch Front, and in Uintah and Duchesne counties in the Uinta Basin in the eastern part of the state. Readings were taken from 2014-2016.

It is the first time Utah has had any counties exceed the federal threshold and fall into what is called "non-attainment" since the early 1990s, Bird said. The federal threshold has decreased several times in 30 years, most recently in 2015.

Ozone levels flare up in the Wasatch Front in the summer, especially on hot and sunny days without rain, Bird said. The main generator is emissions from cars. His agency already issues "calls to actions" on the worst days urging residents to limit car use and avoid exercise from 10 a.m.- 4 p.m.

One challenge for state officials is that residents aren't as aware of summer pollution problems compared to the northern Utah's winter pollution problems, Bird said. During the winter inversions, cold, stagnant air settles in the bowl-shaped mountain basins and traps emissions, creating a brown, murky haze.

"People are very familiar with winter inversions because they can see it, they can taste it, they can feel the impacts," Bird said. "The public typically isn't as aware during the summer time because ozone is odorless and colorless so it looks like a beautiful summer day.

The ozone problem in the Uinta Basin is more unique since it comes during the winter at the same time that the Salt Lake City area experiences winter inversions that create small particulate pollution, Bird said.

Research shows the ozone levels rise in the Uinta Basin based primarily on emissions from the oil and gas activities and worsen on winter days when the sun reflects off the snow, he said. The state has already implemented several rules to reduce pollutants.

The Center for Biological Diversity on Tuesday questioned why President Donald Trump's administration is approving oil and gas leases in the Uinta Basin when they say it is clear ozone levels are dangerous.

"Allowing oil and gas development in areas already choking on pollution is making a bad situation much worse," said Diana Dascalu-Joffe, a senior attorney.